The present invention relates generally to a vehicle skid control system, and more particularly relates to a skid control system embodying protection against a hydraulic failure.
U.S. Pat. No. 4,130,322, for a SINGLE GAIN SKID CONTROL VALVE AND SKID CONTROL SYSTEM, owned by the same Assignee as the present application, discloses and claims a skid control system for controlling the brake pressure when the input metered pressure is in excess of a predetermined control pressure. The control pressure for such a valve is generated in response to a control signal generated by the skid control circuitry of the system.
More specifically, said patent discloses a system including a two-stage control valve wherein the first stage is a hydraulic amplifier for producing a control pressure in response to an input control current. The second stage of the valve is a pressure follower stage for controlling the hydraulic pressure applied to at least one brake port in response to the pressure difference between the control pressure and a metered supply pressure. The supply pressure for the first stage is obtained from a substantially constant pressure source of hydraulic power and is independent of the metered supply pressure variations. The substantially constant pressure source for said valve therefore allows the control pressure to be independent of the metered supply pressure variations under normal skid control operating conditions.
The present invention relates to an improvement in the skid control valve and system disclosed and claimed in said U.S. Pat. No. 4,130,322 to protect the braking and skid control system against hydraulic failure. More specifically, the present invention protects the valve and system against a failure mode wherein the hydraulic pressure from the above-described constant pressure source is substantially reduced or eliminated. In accordance with this invention, the metered supply pressure source is connected in fluid communication with the first stage of the control valve and with the constant pressure source through a series of hydraulic check valves. The check valves and the system are arranged so that the predetermined control pressure generated by the constant pressure source determines the skid-control characteristics of the system, and the valve functions as a single-gain control valve, during normal skid control operating conditions. During such normal operations the check valves isolate the metered pressure source from the constant pressure source and the first stage of the valve, so that the supply pressure of the first stage is independent of the metered supply pressure variations.
Different operating characteristics prevail in the skid control system and valve in the event of a failure in the constant pressure control source. In such a failure mode, the check valves and system function to isolate the failed constant pressure source from the first stage of the valve, and to simultaneously connect the valve first stage with the metered pressure source. By such an arrangement, the valve will function as a multi-gain control valve during the failure mode, and will prevent the applied brake pressure from being reduced drastically, or from going to zero, when metering pressure is applied to the control valve despite failure of the source of constant hydraulic pressure.
The illustrated embodiment of the control valve and system in accordance with this invention includes a two-stage valve for controlling a single vehicle wheel. It will be appreciated that this invention is readily adaptable to other arrangements such as for dual control valves for the differential braking of a pair of wheels.